The delayed coking process is an established petroleum refinery process which is used on very heavy low value residuum feeds to obtain lower boiling cracked products. It can be considered as a high severity thermal cracking or destructive distillation and may be used on residuum feedstocks containing nonvolatile asphaltic materials which are not suitable for catalytic cracking operations because of their propensity for catalyst fouling or for catalyst deactivation by their content of ash or metals. Coking is generally used on heavy oils, especially vacuum residua, to make lighter components that can then be processed catalytically to form products of higher economic value. In the delayed coking process, the heavy oil feedstock is heated rapidly in a tubular furnace from which it flows directly to a large coking drum which is maintained under conditions at which coking occurs, generally with temperatures above about 450.degree. under a slight superatmospheric pressure. In the drum, the heated feed decomposes to form coke and volatile components which are removed from the top of the drum and passed to a fractionator. When the coke drum is full of solid coke, the feed is switched to another drum and the full drum is cooled and emptied of the coke product. Generally, at least two coking drums are used so that one drum is being charged while coke is being removed from the other.
In order to bring the feedstock up to the required temperature and to conserve process heat, the feedstock is usually charged to the base of the fractionator tower which receives the overheads from the coke drum. The feed to the furnace is taken from the bottom of the fractionator or "combination" tower and the products of the coking process, including heavy coker gas oil, light coker gas oil and coker gasoline are removed from higher levels in the tower. The use of the tower bottoms as the feed for the coker furnace has three main objectives. First, heavy fractions which are recycled through the unit will be further cracked to lower boiling products which have greater utility even though the yield of coke ("coke make") is increased by this recycling; second, the metals content of the products is reduced as the coke make increases because the metals tend to accumulate in the coke; third, use of the recycle as diluent tends to reduce coking in the furnace. Coking in the furnace is a significant problem in delayed coking operations because although the yields of coke and gas may be reduced by operating the coking drums at higher temperatures, the higher temperatures which are required in the furnace to provide them, lead to excessive fouling in the tubes of the furnace, with a concommitantly greater maintenance requirement to clean the furnace tubes. Furnace fouling may be reduced by using an inert gas stripper, usually steam, but even then the practical limitations on furnace conditions generally constitute the principal impediment to improved operation of the coker.
Present trends in the petroleum refining industry are making it more and more desirable to increase the yield of lighter products, especially gasoline and distillates, from residual products which themselves are becoming heavier and more difficult to process. This requires a significant increase in residual oil upgradng capacity but because this generally requires major capital expenditure, it would be desirable to find some way of increasing the yield of lighter products using existing equipment. At the present, most delayed coker units are limited by the coke make, that is, by the amount of coke which they produce relative to the yield of cracked products. Although, as mentioned above, the yield of cracked products may be increased by operating at higher temperatures, this is generally not practicable because of the increased downtime required for furnace maintenance. Therefore, any improvement in the delayed coker process should preferably be accomplished without the necessity of operating under conditions which lead to increased furnace fouling and generally this will mean that increases in furnace temperature will normally have to be avoided.
One shortcoming of existing delayed coking technology is that with the heavier crudes now being employed in refineries, relatively large coke yields (of the order of 30 to 40 weight percent) are obtained, with a nonselective yield distribution of relatively low quality, refractory liquid products. The yield distribution is, of course, difficult to control in a purely thermal operation with a given type of feed and therefore offers only a limited potential for improvement. However, the large coke yield and the quality of the liquid product can be attributed to the use of the fractionator or combination tower in which the feedstock is directly heat exhanged with the vaporous effluent from the coker drums. Although this serves to conserve process heat, it also results in the heaviest components of the coker effluent being condensed and returned as recycle to the furnace, generally in amounts which range from 5 to 40 percent of the fresh coker feed, depending on the operational and heat requirements of the particular unit. Although the recycle is highly refractory and, as previously mentioned, tends to reduce coking in the furnace, it nevertheless produces a significant amount of coke so that the final coke yield is increased. Furthermore, the liquid products derived from the heavy recycle tend to be more refractory and of lower quality than the liquid products from fresh feed of the same boiling range.
One proposal for reducing the coke yield in a delayed coker unit is set out in U.S. Pat. No. 4,455,219 which modifies the conventional delayed coking process by reducing the amount of heavy recycle which is returned to the furnace and adding an additional, lighter feedstock component, either from the coker fractionator or from some other source. In this process, the amount of heavy coker gas oil which is returned to the lower section of the fractionator tower is held to the minimum amount necessary for operation of the fractionator, with the balance delivered as product from the unit. This results in a decrease in the amount of recycle, the deficiency being made good by added light distillate which is introduced into the feedstock before it is charged to the base of the fractionator. This proposal does not, however, deal effectively with the problem of the quality and distribution of the liquid products even though some decrease in coke make might be obtained. The reason for this is that only the lighest portion of the heavy recycle stream is removed. The heavier components are returned in the normal way and continue to participate in the process, with the undesirable effects alluded to above. There remains, therefore, a continuing need for improvements in the delayed coking process.